To provide a film thickness measuring device capable of measuring the thickness of a thin film on a substrate during the production process of indication element and the like.

A light radiation means 3 radiating light to a thin film 2 on a substrate 1, an observation wavelength forming means 7 forming an observation wavelength from the light having passed the thin film 2, a light intensity detection means 6 detecting the light intensity of a specific wavelength formed by the observation wavelength forming means 7, and a film thickness detection means 9 detecting the film thickness of the thin film based on the light intensity detected by the light intensity detection means 6 are provided. Light is cast on the thin film 2 on the substrate 1 with the light radiation means 3, a specific observation wavelength only is selected from the light having passed the thin film 2 with the observation wavelength forming means 7, the light intensity of the specific wavelength is detected with the light intensity detection means 6 and based on the light intensity, the film thickness of the thin film is detected with the film thickness detection means 9.